There is great interest in the discovery of Li/Na-ion cathode materials with capacity exceeding the limitation of conventional intercalation-based oxide cathodes. One plausible but challenging path is to reversibly use the charge compensation of both lattice oxygen redox and transition metal (TM) redox. Here, we report that lattice oxygen redox alone contributes over 190 mA h g –1 charge capacity (cut-off at 4.65 V vs. Na +/Na) for the newly synthesized P3-type Na 2/3Mg 1/3Mn(IV) 2/3O 2. Similar amounts of discharge capacity are reversibly achieved. The discharge capacity exceeds 220 mA h g –1 when Mn 3+/Mn 4+ redox is partially used in addition to the oxygen redox reaction. This represents one of the highest energy density sodium-ion cathodes with superior low-cost. Our results reveal that cations with strong ionic bonding nature with oxygen (such as Mg 2+) are very effective in inducing the reversible oxygen redox reaction. Furthermore, we also identified the origin of voltage hysteresis to be a P3-to-O3 phase transition in concomitance with Mg 2+ migration, suggesting further structure design that reduces the structure transition induced cation migration is critical for increasing the energy efficiency of the oxygen redox reactions.

Developing sodium-ion batteries for large-scale energy storage applications is facing big challenges of the lack of high-performance cathode materials. Here, a series of new cathode materials Na 0.66Co xMn 0.66–xTi 0.34O 2 for sodium-ion batteries are designed and synthesized aiming to reduce transition metal-ion ordering, charge ordering, as well as Na+ and vacancy ordering. An interesting structure change of Na 0.66Co xMn 0.66–xTi 0.34O 2 from orthorhombic to hexagonal is revealed when Co content increases from x = 0 to 0.33. In particular, Na 0.66Co 0.22Mn 0.44Ti 0.34O 2 with a P2-type layered structure delivers a reversible capacity of 120more » mAh g -1 at 0.1 C. When the current density increases to 10 C, a reversible capacity of 63.2 mAh g -1 can still be obtained, indicating a promising rate capability. The low valence Co 2+ substitution results in the formation of average Mn 3.7+ valence state in Na 0.66Co 0.22Mn 0.44Ti 0.34O 2, effectively suppressing the Mn3+-induced Jahn–Teller distortion, and in turn stabilizing the layered structure. X-ray absorption spectroscopy results suggest that the charge compensation of Na 0.66Co 0.22Mn 0.44Ti 0.34O 2 during charge/discharge is contributed by Co 2.2+/Co 3+ and Mn 3.3+/Mn 4+ redox couples. This is the first time that the highly reversible Co 2+/Co 3+ redox couple is observed in P2-layered cathodes for sodium-ion batteries. This finding may open new approaches to design advanced intercalation-type cathode materials.« less